Functionalization of hexa-peri-hexabenzocoronenes: Investigation of the substituent effects on a superbenzene

We have demonstrated that the iridium-catalyzed direct borylation of hexa-peri-hexabenzocoronene (HBC) enables regioselective introduction of boryl groups to the para-, ortho-, and meta-substituted HBCs in high yields. The boryl groups have been transformed into various functionalities such as hydroxy, cyano, ethynyl, and amino groups. We have elucidated that the substituents significantly influence the photophysical properties of HBCs to enhance fluorescence quantum yields. DFT calculations revealed that the origin of the substituent effect is the lift in degeneracy in the frontier orbitals by an interaction with electron-donating and electron-withdrawing substituents at the para- and ortho-positions. The change in molecular orbitals results in an increase of the transition probability from the S₀→S₁ states. In addition, the two-photon absorption cross-section values of para-substituted HBCs are significantly larger than those of ortho- and meta-substituted HBCs.